Which action will allow the developer to relate records in the data model without knowing the Salesforce ID?

A developer needs to join data received from an integration with an external system with parent records in Salesforce. The data set does not contain the Salesforce IDs of the parent records, but it does have a foreign key attribute that can be used to identify the parent.

Which action will allow the developer to relate records in the data model without knowing the Salesforce ID?
A . Create a custom field on the child object of type Foreign Key
B . Create and populate a custom field on the parent object marked as Unique
C . Create and populate a custom field on the parent object marked as an External I
E . Create a custom field on the child object of type External Relationship.

Answer: C